1. Field of the Invention
The present invention relates to an optical device including a volume phase type hologram optical element formed on a substrate, an image display device that utilizes the optical device and a head mounted display (hereinafter also referred to as an HMD) equipped with the image display device.
2. Description of Related Art
Conventionally, there are proposed various types of image display devices that conducts image light from a display element (e.g., a liquid crystal display element) to the observer's pupil (of the eye) via an ocular optical system. In this image display device, the image light or external light may propagate through an optical path other than a regular optical path inside the ocular optical system and reach the observer's pupil as flare light or ghost light. In this case, quality of the image (a picture or an external image) to be observed by the observer may be deteriorated.
Therefore, a device described in Patent Document 1, listed below, uses a shading plate disposed between the ocular optical system made up of an eccentric prism and the observer's pupil so that the flare light or the ghost light is shaded by the shading plate. Thus, the quality deterioration of the image observed by the observer is avoided.
The patent document mentioned above is as follows:
Patent Document 1: JP-A-9-65245
It is supposed that image light from the display element is composed of a center beam having high light intensity and a peripheral beam having low light intensity. Then, it can be said that the center beam has high optical performance because it has little aberration in the optical system of the image display device. On the contrary, the peripheral beam cannot be said to have high optical performance because it has much aberration in the optical system. Therefore, it is considered that if the shading plate such as described in Patent Document 1 is provided to the ocular optical system on the observer side so that only the peripheral beam of the image light from the display element can be blocked, a high quality image formed only by the beam having high optical performance would be provided to the observer.
In this case, however, the necessity of the shading plate that is an optical element disposed just in front of the eye of the observer would cause an increase in size of the device due to an increase of the thickness thereof, resulting in an increase of cost of the device. In addition, if the ocular optical system is used as a combiner of image light and external light, a field of view for the observer to observe an external image would be blocked partially by the shading plate. As a result, a range for the observer to observe the external image would be narrowed.